Unraveling the Structure and Properties of High-Concentration Aqueous Iron Oxide Nanocolloids Free of Steric Stabilizers

Aqueous colloids with a high concentration of nanoparticles and free of steric stabilizers are prospective soft materials, the engineering of which is still challenging. Herein, we prepared superparamagnetic colloids with very large, up to 1350 g/L concentration of 11 nm nanoparticles via Fe²⁺ and Fe³⁺ coprecipitation, water washing, purification using cation-exchange resin, and stabilization with a monolayer of citrate anions (ζ potential of diluted dispersions about -35 mV). XRD, XPS, Mössbauer, and FTIR spectra elucidated the defective reverse spinel structure of magnetite/maghemite (Fe₃O₄/γ-Fe₂O₃) with a reduced content of Fe²⁺ cations. The viscosity increases with nanoparticle concentration and depends also on the nature of citrate salt, being one order of magnitude lower for lithium than sodium and potassium as counter-cation. SAXS/USAXS curves show power-law behavior in the scattering vector range between 0.1 and 0.002 nm^-1, suggesting that particles interact forming fractal clusters, which are looser for Na⁺- and denser for Li⁺-citrate stabilizers (fractal dimensions of 1.9 and 2.4, respectively). In parallel, ATR-FTIR found increasing proportions of symmetric O-H stretching vibrations of ice-like interfacial water in the concentrated colloids. We hypothesize that the clusters arise due to the attraction of like-charge particles possibly involving the water shells and hydration of counter-cations; overlapping the clusters and transition to continuous non-Newtonian phases is seen at viscosity vs concentration plots at 700-900 g/L. The results shed new light on the structure of very concentrated nanocolloids and pave the way for their manufacturing and tailoring.